1. Field of the Invention
The present invention relates to an optical system suitable for being used as an image pickup optical system to be used in an image pickup apparatus such as a silver-halide film camera, a digital still camera, a digital video camera, a monitoring camera, and a TV camera.
2. Description of the Related Art
In recent years, an image pickup optical system to be used in an image pickup apparatus has been required to have high optical characteristic, a wide field angle, and a large aperture ratio.
Japanese Patent Application Laid-Open Nos. H11-30743 and 2009-109723 disclose a retrofocus type lens which includes a first lens unit having a negative refractive power and a second lens unit having a positive refractive power in order from an object side to an image side.
A structure in which refractive power arrangement is made asymmetric with respect to an aperture stop is known as a structure for realizing an image pickup optical system having a wide field angle and a large aperture ratio. However, when the refractive power arrangement is made asymmetric with respect to the aperture stop, many various aberrations (specifically, high-order various aberrations) are generated. A coma in a sagittal direction (in a direction concentric with respect to an optical axis) is known as one of the high-order various aberrations and is called a sagittal coma flare.
In order to realize the image pickup optical system having the wide field angle and the large aperture ratio, it is important to satisfactorily correct the sagittal coma flare. To satisfactorily correct the sagittal coma flare while the large aperture ratio and the wide field angle are realized to obtain the high optical characteristic in the image pickup optical system including two lens units, it is important to appropriately set the refractive power and lens structure of the first lens unit, the refractive power and lens structure of the second lens unit, and the like.